Editing Siphuncle (section)

== Function ==
[[File:Siphuncle.svg|thumb|200px|Simplified structure and mechanism of cephalopodic siphuncle.|left]]

The siphuncle is used primarily in emptying water from new chambers as the shell grows.<ref name="Mutvei2007">{{Cite journal
 | last1 = Mutvei | first1 = Harry
 | last2 = Zhang | first2 = Yun-bai
 | last3 = Dunca | first3 = Elena
 | year = 2007
 | title = Late Cambrian Plectronocerid Nautiloids and Their Role in Cephalopod Evolution
 | journal = Palaeontology
 | volume = 50
 | issue = 6
 | pages = 1327–1333
 | doi = 10.1111/j.1475-4983.2007.00708.x
 | doi-access = 
 | bibcode = 2007Palgy..50.1327M
 }}</ref> To perform this task, the cephalopod increases the saltiness of the [[blood]] in the siphuncle, and the water moves from the more dilute chamber into the blood through [[osmosis]]. At the same time gasses, mostly [[nitrogen]], [[oxygen]], and [[carbon dioxide]], diffuse from the blood in the siphuncle into the emptying chamber. This is not a form of active pumping: the gas moving into the chamber is a passive process. Most energy is expended through the absorption of water from the chamber.

Removing water from the chambers of the shell reduces the overall [[density]] of the shell, and thus the shell behaves as a flotation device comparable to the [[swim bladder]] in bony [[fish]]. Typically, cephalopods maintain a density close to that of [[sea water]], allowing them to keep a stable buoyancy with minimal effort. In the geologic past, many cephalopods grew to an [[cephalopod size#Extinct taxa 2|enormous size]] (perhaps approaching ten meters in length) thanks to this.

Generally, the siphuncle is unable to provide a way to change the density of shell rapidly and thus cause the animal to rise or sink at will; rather, the animal must swim up or down as required.

Cephalopods with a wider siphuncle have a higher rate of metabolic activity.<ref>{{cite journal|doi=10.1007/BF03043304|title=The size of the siphuncle in cephalopod evolution|journal=Senckenbergiana Lethaea|volume=83|pages=39–52|year=2003|last1=Kröger|first1=Björn|issue=1–2 }}</ref>